Sharpening Part 21 – The Bulging Bevel

For everybody in their busy lives, you need to invest in sharpening your tools, and you need to invest in longevity.

Ryan Holmes

In the previous post in this series about sharpening tools we looked at why and how to true the ura, the hollow-ground area on Japanese chisel and plane blades. This post will focus on the opposite side of the wedge that is a cutting edge: the bevel. This discussion is relevant to all plane and chisel blades, not just Japanese tools.

Preface

Before we dive in, I need to clarify something.

Some of our Gentle Readers have been blessed with the opportunity to learn about tools from accomplished Japanese craftsmen, as was I, or have figured them out on their own. If you find this or other posts boring, please remember this blog’s primary purpose is to provide instruction to our Beloved Customers who have not had similar opportunities.

These Beloved Customers are located in many countries and range in experience from newbies to professional woodworkers, so I try to include both advanced information for the professionals, and detailed explanations so newbies can keep up. Consequently, these articles are sometimes long and wordy. I humbly request your kind indulgence on behalf of those who may benefit.

Investing in Longevity

The quote above by Mr Holmes is applicable to the all the principles of sharpening I have described in this series of posts so far. He is a computer dude, not a contractor, joiner or furniture maker, but it is no coincidence he chose to use handtool terminology: it is encoded in human DNA.

His first point is a self-evident admonition, but what about this “investing in longevity” stuff? By definition, an investment is an expenditure of time, resources and/or effort intended to produce a return greater in value than the expenditure. Then how do we go about investing in the longevity of our chisels and planes, and what return should we expect?

While simply grinding sharp edges on our tools helps with making things from wood, I don’t see it as an investment in tools. Rather, if we train ourselves in professional sharpening techniques, and use those techniques to maintain our tools so they function more efficiently and last longer, we can hope to obtain a real-world return we can quantify financially. The investment I encourage you to make is not in things, therefore, but in your own skills.

The Pros and Cons of the Bulging Bevel

The “bulging bevel,” as I call it, is a deformation too frequently seen in plane, chisel and knife blades. It is simply a cutting edge bevel that is protruding and convex instead of flat. In most cases a bulging bevel can make it difficult to properly sharpen a blade adequately, so it deserves our attention. Most bulging bevels are born unintentionally and are harmful, but some are hatched with a purpose in mind. Let’s examine the pros and cons, and throw in some scientific results just for fun.

The geometry of the bulging bevel is clearly superior in a few applications such as carving chisels and knives used in a gouging, scooping motion where a rounded bevel provides better control. Another is chisels used for cutting large and deep mortises where a rounded bevel helps pop out waste easier. Only timber framers cut these kind of mortises, however, and most of them use machines to at least rough out the mortises.

Hidari no Ichihiro 42mm Oiirenomi. Nothing obese about this sweetheart.

Our Beloved Customers are, without exception, extremely intelligent people, so right now some of you are no doubt saying to yourself: “Self,” (that’s what they call themselves when they silently cogitate profound matters) does a rotund bevel make my blades sharper or duller?” Let us consider some scientific results.

When I was a grad student in Japan, a fellow student wrote his thesis on the efficacy in plane blades of the bulging bevel versus the flat bevel. He developed experiments, fabricated testing apparatus, and used scientific methodology and microscopic photography yielding indisputable results. We repeated some of his experiments, discussed his research, and pored over photographs and fondled shavings late into the evenings at his lab in Building 11 at University of Tokyo Hongo campus as I drank coke and he drank sake. I’m not sure he made it home some evenings.

The conclusion he reached was that, from the viewpoint of the wood, and based on the classic sharpness test of cutting rag typing paper, there is no difference in the cutting performance between flat and bulging bevels, so long as two conditions are met: (1) Both types are sharpened to the same bevel angle and same degree of sharpness; and (2) The bulge is not so large as to interfere with the cut. The “same degree of sharpness” condition in proviso 1 is critical to this discussion.

Let’s examine the cutting edge closely. It’s effective scope is only the last few microns (μ) or so of the blade’s width at the extreme edge. 1μ=one millionth of a meter. A human hair is 90μ in diameter. We need to precisely repair and polish this narrow strip of steel using our sharpening stones, but remember that working anything beyond this strip contributes nothing to making the blade sharp.

Here’s an important point we can learn from a careful examination: Given the same number of strokes to the same blade on the same stones over the same amount of time, it is difficult to make a bulging bevel as sharp as a flat bevel, unless one spends the time to use a sharpening machine and jigs as my grad school friend did in his research room.

But the most important point, and one I want you to grasp with both hands and feet and all your teeth is that the time expended and amount of stone consumed when sharpening by hand to a set level of sharpness at the last critical microns of a bulging bevel’s cutting edge is huge compared to a flat bevel. Sharpening using machines and/or honing jigs takes even longer.

In addition to time and cost, another factor we need to consider is certainty, because if we are going to invest the time and stones to sharpen a tool, we need to be sure it will consistently achieve the same level of sharpness every time. Unfortunately, the sharpness of the bulging bevel is uncertain because, instead of guiding the blade to ensure consistent contact between steel and stone at the critical location on the cutting edge, the shape of the bulging bevel causes a significant number of strokes to be wasted on polishing a mound of metal that does nothing to make the blade sharper, but is simply in the way. Not convinced?

Consider the undeniable fact that, despite your best efforts, this irrelevant lump causes the blade to rock around on the stone’s surface like a boat over ocean swells, with the result that, given a fixed number of strokes, a high percentage of those strokes end up polishing the bulge instead of the cutting edge. This is important because, once again, the last micron of the blade is the only part that actually does any cutting, not the bulge.

Please don’t misunderstand. I’m not saying that you can’t create a fiendishly sharp edge on a blade with a bulging bevel. I’m also not saying that, within reasonable parameters, a convex bevel cuts less efficiently or dulls quicker than the same blade with a flat bevel. It absolutely doesn’t, as my colleague’s research showed. Allow me to restate and summarize the facts so there is no confusion.

It takes longer to create a given level of sharpness at the extreme cutting edge of a bulging bevel than a flat bevel, all else equal;

It consumes more sharpening stone to achieve a given level of sharpness at the extreme cutting edge of a bulging bevel compared to a flat bevel, all else equal; and

There is greater uncertainty about the actual degree of sharpness achieved at the blade’s extreme cutting edge when sharpening a bulging bevel by hand compared to a flat bevel, all else equal.

If you doubt these statements, you must find the truth yourself. Buy or borrow a quality loupe or microscope with enough magnification to detect the scratches left by your usual finishing stone. Start with a dull blade with a truly flat bevel, sharpen it freehand using a set number of strokes, and observe the scratches at the last few microns of the cutting edge with your microscope. Then test the blade’s sharpness with your skin or fingernail. Next, repeat this test with a dull blade with a rounded bevel using the exact same sharpening tools and procedures and the same number of strokes. Once again, observe the scratches and test the sharpness. My grad school friend and I performed this side-by-side experiment at the University of Tokyo several times, with consistent results. Actually, it was a bet and I won. He had to buy the drinks and snacks for a month.

The Causes of Bevel Obesity

Besides pernicious pixies, the most common cause of bevel bulge is simple carelessness, which you can take steps to avoid once you realize the causes.

It is human tendency to try to stabilize the blade’s bevel on the stone while sharpening by applying more pressure on the rear half of the bevel, resulting in the rear half of the bevel (which is all soft jigane in the case of plane blades, and mostly soft jigane in the case of chisels) being abraded quicker than the front half (which contains the harder steel lamination), causing the bevel angle to gradually decrease or even become rounded. Even the best craftsmen make this mistake sometimes.

To avoid this tendency, train yourself to focus pressure on the front half of the bevel closest to the cutting edge. At first, you may overbalance and dig the cutting edge into the stone a few times, but with practice and attention, it will become second nature. It is almost a meditative process. Every professional woodworker worth his salt must learn this skill.

There is nothing wrong with making mistakes when learning a muscle memory skill like freehand sharpening, but too many people can’t be bothered to learn, and then become frustrated when their skills don’t improve immediately. In the end, they become defensive, and twist themselves into knots defending their inadequate techniques. Patience, grasshopper.

BTW, don’t forget to use your handy dandy brass bevel gauge to both check the bevel angle while sharpening and to keep those piratical pixies away.

Another cause of the tumescent bevel is the use of secondary bevels or micro-bevels. We’ll look at these aberrations in the next post in this series.

To make multiple bevels work one almost must use a sharpening or honing jig of some sort. Many allow sharpening jigs to become a substitute for real sharpening skills they didn’t bother to learn. Such jigs can become, in effect, training wheels those who rely on them never grow out of. So sad.

Conclusion

I encourage you to “invest in longevity” with regards to your tools in three ways:

First setup your planes and chisels properly so they will provide you with long, reliable and efficient service. Setting up chisels improves not only their longevity but in many case their performance too, strange though it may seem. I will post articles about setting up and maintaining Japanese planes in the future.

Second, true the ura of your plane and chisel blades efficiently without reducing their useful lifespan needlessly, as described in previous posts; and

Third, invest in yourself by developing and honing the hand skills necessary to sharpen your blades quickly and efficiently while wasting only the absolute minimum of valuable time, steel and stone.

Please master the ancient and bedrock-basic skill of freehand sharpening. All it takes is an understanding of correct principles, followed by concentration and practice; The rest will follow. I promise. “This is the way.”

We will look at other causes of bevel obesity in the next post in this series.

If you have questions or would like to learn more about our tools, please use the questions form located immediately below. Please share your insights and comments with everyone in the form located further below labeled “Leave a Reply.” We aren’t evil Google or incompetent facebook and so won’t sell, share, or profitably “misplace” your information.

14 thoughts on “Sharpening Part 21 – The Bulging Bevel”

This is far from being boring, if people think so then they can go elsewhere cause last time I check you don’t force anyone to read on!!! I’ve been at this for a while now but still learn things everyday, keep an open mind and grab all available information cause the more you read the more you can select what is worth keeping and what is not!
I would say that your writing is really entertaining, so just for that people should read on even if they “know it all”!!

I personally try to keep as flat a bevel as I can and sometimes if I have pieces wood that is being tough on edges, I will on the last stroke or two on the finishing stone “lift” the top of the blade( the part we tap on to adjust it in the dai) to create a “nano” bevel(way smaller than the micro bevel showed every where). I often forget to do this…. I got this info from a friend and it seems to work pretty good! I know some people will put a “back bevel” (really tiny) but I find it more trouble than benefice!!

Indeed, lifting the last couple of strokes to create a “nano bevel (I like that term!) works if it doesn’t go too far. Fact of the matter is that freehand sharpening almost always produces something similar anyway. Problem is if I start writing about it people will immediately let it get out of control. Human nature. The nano bevel on the ura side is an unkind thing to do to a blade and will bite those who do it in the ass.

The most important piece of advice I can give is, relax! Don’t hold your chisel or plane blade like you’re holding on for dear life. Put your fingers as far down as possible on the bevel, right there where the jigane ends and the hagane starts and relax the grip with your other hand. This keeps enough pressure focussed on the hagane to avoid the bevel angle becoming shallower, and ensures there’s enough contact with the jigane to keep the bevel flat. The hand with which you hold the chisel should really only be steering the blade and hold it up. Uptight sharpening will never end in good results.

Having said this, I use nano bevels a lot on my atsunomis. I lift the blade a few degrees and do a couple of light strokes on my finishing stone to finish off the edge. This is especially beneficial when I’m working with hard white oak.

But for those reading this, please don’t get carried away with “nano bevels” or they will become micro-bevels and that way lies madness. If you find you need to always make a small bevel on your plane or chisel’s bevel to stop tearout or to prevent the blade from chipping, you should adjust the angle of the bevel anyway.

I wouldn’t really even call it a nano bevel, more like a microscopic bevel. I find that lifting a few degrees and only using the weight of the blade for the last 4 or 5 strokes makes the edge just a little bit smoother and tougher. I really don’t want a 35 degree or higher bevel angle as this also reduces the efficiency.

Stan, could you say a few words about why Japanese chisels and plane blade bevels are not usually hollow ground (the opposite of bulged) on a grinding wheel before honing? Is it because that would remove too much of the soft steel and leave the harder steel not as well supported?

Henk is exactly correct. Chisels especially need their hard steel layer supported by jigane, which grinding can literally undermine. And the heat of an electric grinder can soften the steel quickly without showing discoloration.

It isn’t impossible to use a grinder, but you need to be painfully careful. It’s a bad habit to get into as a routine step in sharpening. We have a warranty at stake, so if people are going to do, it will not be with my blessing.

More meat behind the hard edge is one reason. The other is that the chisels and kanna blades Stan sells are probably tempered around 110-120 degrees Celsius. Stan probably knows the exact tempering temperature. Anyway, a bench grinder produces quite a bit of heat and before you know it you’ve gone over the tempering temperature and the blade is ruined. Instead of a 66Hrc blade you end up with a 58Hrc or lower blade.

Thank you for this, Stan. One of the first Japanese chisels I bought had a bulging bevel. I tried to ignore it and being unaware of what you explained above I could have ignored it. But it didn’t sit right with me. I hated the look of it. I knew that bevel, any bevel, should be flat. Usually I sharpen by hand but I knew this would take some doing and I had to get the angle right so I clamped the chisel in my Veritas honing guide and went at it until that puppy was truly flat. I sweated like a gypsy with a mortgage. But I got it right and now I love the thing. It’s my go-to chisel in that size. Now it’s easy to sharpen by hand and a pleasure to use. Thanks for confirming what I knew by sight was something I had to fix.